"We're continuing work to mitigate the attack and service has been available again since about 6:15 p.m. PT," Ronda Scott, a spokeswoman for Evernote, wrote in an email.

DOS attacks involve sending huge amounts of traffic to a server in order to slow it down or send it offline. This can involve hijacked PCs, where owners are unaware of their involvement, in what is called a botnet.

While the attack did disrupt the company's online services, Scott told the BBC that users accounts were not infiltrated.

"As is the nature of DoS attacks, there was no data loss, and no accounts were compromised."

Since the company's launch in 2008, Evernote has undergone major growth and last month announced that it had reached 100 million users. However, while the most recent security scare is the first successful DOS attack in some time, the company did review its security systems last year after hackers managed to access users' email-addresses and passwords.

Earlier today, it was also announced that news aggregator Feedly had experienced a similar attack. While Feedly and Evernote work closely together, it has not yet been confirmed if the two attacks are connected.